After arguing over where to camp for summer vacation, Nobita, Doraemon, Shizuka, Gian and Suneo settle on the sea floor, breathing underwater thanks to one of Doraemon's gadgets and getting around in Buggy, a talking underwater vehicle with a soft spot for Shizuka. When Gian and Suneo sneak off across the Atlantic after a sunken treasure ship in the news, the gadget's protection begins to fade. Strange craft appear, a giant squid wrecks the camp, and the whole group ends up captured by people of the undersea kingdom of Mu.